Retention controls on-shelf hypoxia in coastal upwelling systems

Abstract:

A maximally simple, coupled physical-biogeochemical box model is used to examine interactions controlling seasonal hypoxia over the continental shelf in an idealized Eastern Boundary upwelling system (EBUS). On-shelf bottom oxygen drawdown is controlled by the balance between flushing with offshore source waters and the degree to which surface particulate organic carbon (POC) production and its subsequent sinking and respiration are retained over the shelf and focused on the bottom. In the model, POC production is controlled by wind-driven upwelling of a generic nutrient to the surface, balanced by respiration and sinking. The degree to which this production occurs over shelf is a balance between the net POC growth rate η, and the upwelling driven shelf flushing rate λ, equivalent to the Ekman upwelling index scaled by the shelf width and surface layer depth. Initial oxygen drawdown follows this exponential rate of surface POC accumulation over shelf (α=η−λ), and so is slower for rapid upwelling (large λ) or nutrient limited conditions (small η). Retention efficiency indicates that up to 40% or more of upwelled nutrients can be retained over the shelf and contribute to oxygen drawdown. Fluctuations in the amplitude of the upwelling forcing can systematically reduce the potential for oxygen depletion below these estimates, especially when surface sinking export is rapid or forcing is strong.